1. Field of the Invention
The present invention is broadly concerned with vertical, multiple-deck continuous batch dryers designed for drying of pellets and other similar products. More particularly, the invention pertains to such dryers and drying methods wherein the dryers have a series of superposed, air-pervious drying decks and an air circulation assembly operable to generate and direct a continuous drying air stream through the decks; a common upright plenum chamber is provided adjacent and in communication with the dryer decks, and the air circulation assembly is designed to pass the air stream into, through and out of the plenum chamber as the air stream passes through respective decks. This allows easy removal of entrained fines within the drying air stream and also permits the user to individually control both the air flow and the percentage of air recirculation through each deck, independently of the other decks.
2. Description of the Prior Art
Multiple-deck vertical continuous batch dryers have been used in the past for drying of pellets and other agricultural and food products. A vertical design allows product transfer between decks with good product separation. Moreover, a higher degree of moisture uniformity is achieved owing to multiple turning of the product as it passes between the vertically spaced decks. The countercurrent design of these dryers (product descends and air flows move upwardly) also gives higher energy efficiencies.
Several different design approaches have been tried in the past with vertical dryers. In one system, each deck assembly is provided with a separate fines collection unit in the form of a cyclone separator, fan and heater. This approach does have the virtue of removing fines at each deck level, thus minimizing the tendency for fines to accumulate on internal dryer components. However, this is a very expensive expedient, with the multiple fines collection units greatly increasing equipment costs and necessary plant space. In another system, only a single collection device is provided at the upper outlet of the dryer. This significantly reduces costs, but does not remove fines at each dryer stage. Thus, it is necessary to have increasing air velocities from bottom to top of the dryer in order to insure that the fines remain entrained in the drying air stream for ultimate separation at the final collector.
Furthermore, both of these prior art approaches suffer from the inability to effectively and efficiently control dryer operation at each deck, independently of the other decks. This means that the air flows (velocities) through each deck cannot be independently controlled, nor can the amount of air recirculation at each deck be controlled.
There is accordingly a real and unsatisfied need in the art for an improved vertical dryer which avoids the high costs associated with multiple collector type dryers, while at the same time giving the same or a better degree of staged fines removal. Also, there is a need for a vertical dryer wherein the individual decks thereof can be independently controlled in terms of airflow velocities and recirculation characteristics.